This invention relates generally to a tapered cutting or milling tool for reaming or milling out collapsed metal tubular members, dents, and mashed in areas of metal tubular members, such as casing, downhole in a well bore. Such tapered milling tools have also been employed for enlarging or opening tubular members as well as removing burrs from the tubular members.
The tapered milling tools have normally been provided with a plurality of blades extending outwardly from the tool body. In most instances, the blades have been fixed to the tapered tool body but in some instances the blades have been mounted on the tool body for outward swinging movement. However, such prior blades have been formed with tungsten carbide cutting fragments or chips embedded in a random pattern in a matrix formed of a suitable powder metallurgy composite material, such as sintered tungsten carbide in a cobalt matrix to provide the cutting surface, and the cutting elements heretofore have not been arranged or constructed to provide a "chip breaker" action for controlling the length of the metal turnings. As an example of such a cutting tool, a tapered body having blades extending from the side of the tool body and utilizing crushed tungsten carbide particles in a matrix have been used heretofore in reaming or milling out collapsed casing downhole in a well bore.
Milling or cutting tools having blades extending from the tool body and formed with the tungsten carbide chips embedded in a matrix have normally utilized blades for taking a relatively small thickness bite from the metal tubular member to be removed which provides a conglomeration of shapes and sizes or metal turnings including fine hair-like turnings of around 6 inches in length, for example. Such turnings tend to curl and internest with each other to provide a so-called "bird nest" or mass which is difficult to remove from the well bore by drilling fluid after being cut from the metal object. A critical factor in obtaining a high rate of penetration through this restricted area within the casing is being able to maintain a sharp predictable cutting structure and to reduce the bearing surface which forms as the cutting structure is worn away. Heretofore a tapered milling tool included blades along its length dressed with a crushed carbide matrix. Each blade was from 1-2 inches in width and, depending on the size of the milling tools, from 2 to 4 feet in length. As the carbide structure wore away, a large flattened surface forms at the junction of the outer blade surface and casing interference point creating a profile that required increasing loads to force the mill deeper into the swaged in area. Oftentimes, numerous mills would be run to ream through a resisted area because of this deterioration of cutting structure.